@JeremyQuick: How do they depend on each other?
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ThomasOct 9 '12 at 17:26

1

For instance, do you know the likelihood of Y given X. If so, you could compute $P(X \cap Y)=P(Y|X)P(X)$. But without any further information, it is difficult to recover the joint probability from the marginals.
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CristianOct 9 '12 at 17:26